The slug generation method is set to ascii but I accidentally typed the slug with an accent. After that I couldn’t enter the edit page of the category.
In the end I fixed the problem entering the rails console and fixing the slug there.
It’s not totally Discourse fault since that depends on the user typing an accent when there shouldn’t be one. But it would be cool if the system checked if the slug typed has only ASCII characters if the slug generation method is set to ascii.